The invention relates to a strain gage transducer with a foil strain gage arrangement secured to a spring. Such transducers include at least one foil strain gage and the respective terminals.
Such transducers are well known in the art. Reference is made, for example, to the "Handbook of Scale Making" by Reimpell-Bachmann, Volume 3, 1966, page 34. The foil strain gage arrangement in this context may comprise a single foil strain gage, or a half bridge, or a full bridge. It is further known from "Home Grown Strain Gage Transducers", by Dorsey, 1976, pages 11 to 13 and page 25, to provide the foil strain gage arrangements for such transducers with adjustable resistances. These resistances or impedances are made of different materials and thus permit the compensation of the temperature characteristic of the null signal as well as of the usuable signal. Further, these resistors permit the null adjustment as well as the calibration of the transducer. The different impedances are short circuited by low resistance conductors which are interruptable. Thus, by interrupting the short circuit conductors, it is possible to electrically include into the bridge circuit the respectively required resistances. However, in this manner only a rather limited number of parameters are adjustable. Further parameters as for example, the creeping or sensing errors due to torsion cannot be compensated according to the prior art. Besides, in this type of prior art arrangement it is not possible to influence the individual foil strain gages.
German Patent Publication No. (DE-OS) 2,831,590 discloses layer or film resistances comprising a main resistor element and additional elements which are short circuited through respective low resistance, interruptable conductor elements. The main resistor elements and the additional elements are connected in series and the resistance value of the additional elements is provided in binary steps. The film resistors are adjusted in such a manner that the conductor sections are interrupted at the required locations, whereby at least portions of the additional resistor elements become effective. By means of this prior art teaching it is possible to quickly adjust film or layered resistors to a desired resistor value with sufficient accuracy and by means of a few interruptions in the conductor element.
In connection with the manufacturing of transducers or pick-ups, so-called creeping is a special problem. Creeping is defined as the change of the transducer output signal as a function of time while maintaining a uniform load on the transducer. Thus, reference is made to FIG. 1 to illustrate the causes of creeping. FIG. 1 shows the expansion of a spring as a function of time and also the strain measured by a foil strain gage applied to the spring as a function of time. At the point of time t.sub.o the spring and thus the foil strain gage shall be loaded with a load represented by a stepped function. Corresponding to the loading the spring and the applied foil strain gage expands by a certain amount S.sub.o as shown in FIG. 1. As time passes, the expansion of the spring increases although the load remains constant. This is referred to as "positive creeping". At the same time the grid of the foil strain gage contracts which is referred to as "negative creeping". Such constriction is basically due to the non-ideal elastic characteristic of the synthetic resin adhesive which is used for securing the foil strain gage to the spring.
The positive creeping of the spring depends on the spring material and on the shape and the size of the spring. The negative creeping of the foil strain gage depends primarily on the number and configuration of the direction reversing zones of the foil strain gage. Thus, it is possible to, for example, substantially reduce the negative creeping of the foil strain gage by increasing the size of the direction reversing zones of the foil strain gage to form so-called "dead-ends". If the absolute value of the negative creeping of the foil strain gage corresponds exactly to the absolute value of the positive creeping of the spring then the two effects compensate each other. However, the creeping is a very complex phenomenon which so far has not been completely determined theoretically and in a quantitive manner. Thus, heretofore it was necessary to provide for each new spring a new strain gage arrangement for compensating the positive creeping of the spring. This is so even if the new spring has a known shape and is merely larger or smaller than a previous spring.
For this purpose a foil strain gage arrangement is especially suitable for which the creeping may be varied as is known, for example, from German Patent Publication No. (DE-PS) 2,049,820. This prior art arrangement comprises at least one foil strain gage with a meandering electrical conductor extending across the direction of strain or expansion. The electrical conductor additionally comprises direction reversing zones in such a manner that the creeping of a defined portion of the foil strain gage compensates the creeping of the other portions of the measuring device, whereby electrical taps are provided at several points of the conductor. In the device according to German Patent Publication No. 2,049,820 the proportion of the creeping compensating grid direction reversing zones and grid ends may be varied by a respective connection of the taps. Stated differently, the foil strain gage arrangement may be adapted to the creeping characteristics of the pick-up or transducer.
However, in such an arrangement it is disadvantageous that the characteristics of the foil strain gage such as the impedance and the sensitivity are varied with the switching-on of additional conductor direction reversing zones. Thus, even after the correct creeping proportion has been switched-on, so to speak, further substantial compensating steps must be taken on the transducer in order to obtain data which are comparable to those initially measured.